GSK3β (glycogen synthase kinase 3β) is a proline directed serine, threonine kinase that plays an important role in the control of metabolism, differentiation and survival. It was initially identified as an enzyme able to phosphorylate and hence inhibit glycogen synthase. It was later recognized that GSK3β was identical to tau protein kinase 1 (TPK1), an enzyme that phosphorylates tau protein in epitopes that are also found to be hyperphosphorylated in Alzheimer's disease and in several taupathies. Interestingly, protein kinase B (AKT) phosphorylation of GSK3β results in a loss of its kinase activity, and it has been hypothesized that this inhibition may mediate some of the effects of neurotrophic factors. Moreover, phosphorylation by GSK3β of β-catenin, a protein involved in cell survival, results in its degradation by an ubiquitinilation dependent proteasome pathway.
Thus, it appears that inhibition of GSK3β activity may result in neurotrophic activity. Indeed there is evidence that lithium, an uncompetitive inhibitor of GSK3β, enhances neuritogenesis in some models and also increases neuronal survival, through the induction of survival factors such as Bcl-2 and the inhibition of the expression of proapoptotic factors such as p53 and Bax.
Recent studies have demonstrated that β-amyloid increases the GSK3β activity and tau protein phosphorylation. Moreover, this hyperphosphorylation as well as the neurotoxic effects of β-amyloid are blocked by lithium chloride and by a GSK3β antisense mRNA. These observations strongly suggest that GSK3β may be the link between the two major pathological processes in Alzheimer's disease: abnormal APP (Amyloid Precursor Protein) processing and tau protein hyperphosphorylation.
Although tau hyperphosphorylation results in a destabilization of the neuronal cytoskeleton, the pathological consequences of abnormal GSK3β activity are, most likely, not only due to a pathological phosphorylation of tau protein because, as mentioned above, an excessive activity of this kinase may affect survival through the modulation of the expression of apoptotic and antiapoptotic factors. Moreover, it has been shown that β-amyloid-induced increase in GSK3β activity results in the phosphorylation and, hence the inhibition of pyruvate dehydrogenase, a pivotal enzyme in energy production and acetylcholine synthesis.
Altogether these experimental observations indicate that GSK3β may find application in the treatment of the neuropathological consequences and the cognitive and attention deficits associated with Alzheimer's disease, as well as other acute and chronic neurodegenerative diseases and other pathologies where GSK3β is deregulated (Nature reviews Vol. 3, June 2004, p. 479-487; Trends in Pharmacological Sciences Vol. 25 No. 9, September 2004, p. 471-480; Journal of Neurochemistry 2004, 89, 1313-1317; Medicinal Research Reviews, Vol. 22, No. 4, 373-384, 2002).
The neurodegenerative diseases include, in a non-limiting manner, Parkinson's disease, tauopathies (e.g. Fronto temporal dementia, corticobasal degeneration, Pick's disease, progressive supranuclear palsy), Wilson's disease, Huntington's disease (The Journal of Biological Chemistry Vol. 277, No. 37, issue of September 13, pp. 33791-33798, 2002), Prion disease (Biochem. J. 372, p. 129-136, 2003) and other dementia including vascular dementia; acute stroke and other traumatic injuries; cerebrovascular accidents (e.g. age related macular degeneration); brain and spinal cord trauma; amyotrophic lateral sclerosis (European Journal of Neuroscience, Vol. 22, pp. 301-309, 2005) peripheral neuropathies; retinopathies and glaucoma. Recent studies have also shown that inhibition of GSK3β results in neuronal differentiation of embryonic stem cells (ESC) and support the renewal of human and mouse ESCs and the maintenance of their pluripotency. This suggests that inhibitors of GSK3β could have applications in regenerative medicine (Nature Medicine 10, p. 55-63, 2004).
Inhibitors of GSK3β may also find application in the treatment of other nervous system disorders, such as bipolar disorders (manic-depressive illness). For example lithium has been used for more than 50 years as a mood stabiliser and the primary treatment for bipolar disorder. The therapeutic actions of lithium are observed at doses (1-2 mM) where it is a direct inhibitor of GSK3β. Although the mechanism of action of lithium is unclear, inhibitors of GSK3β could be used to mimic the mood stabilising effects of lithium. Alterations in Akt-GSK3β signaling have also been implicated in the pathogenesis of schizophrenia.
In addition, inhibition of GSK3β could be useful in treating cancers, such as colorectal, prostate, breast, non-small cell lung carcinoma, thyroid cancer, T or B-cell leukaemia and several virus-induced tumours. For example, the active form of GSK3β has been shown to be elevated in the tumors of colorectal cancer patients and inhibition of GSK3β in colorectal cancer cells activates p53-dependent apoptosis and antagonises tumor growth. Inhibition of GSK3β also enhances TRAIL-induced apoptosis in prostate cancer cell lines. GSK3β also plays a role in the dynamics of the mitotic spindle and inhibitors of GSK3β prevent chromosome movement and lead to a stabilisation of microtubules and a prometaphase-like arrest that is similar to that observed with low doses of Taxol. Other possible applications for GSK3β inhibitors include therapy for non-insulin dependent diabetes (such as diabetes type II), obesity and alopecia.
Inhibitors of human GSK3β may also inhibit pfGSK3, an ortholog of this enzyme found in Plasmodium falciparum, as a consequence they could be used for the treatment of malaria (Biochimica et Biophysica Acta 1697, 181-196, 2004). Recently, both human genetics and animal studies have pointed out the role of Wnt/LPR5 pathway as a major regulator of bone mass accrual. Inhibition of GSK3β leads to the consequent activation of canonical Wnt signalling. Because deficient Wnt signalling has been implicated in disorders of reduced bone mass, GSK3β inhibitors may also be used for treating disorders of reduced bone mass, bone-related pathologies, osteoporosis.
According to recent data, GSK3β inhibitors might be used in the treatment or prevention of Pemphigus vulgaris.
Recent studies show that GSK3beta inhibitor treatment improves neutrophil and megakaryocyte recovery. Therefore, GSK3beta inhibitors will be useful for the treatment of neutropenia induced by cancer chemotherapy.
Previous studies have shown that GSK3 activity decreases LTP, a electrophysiological correlate of memory consolidation, suggesting that inhibitor of this enzyme may have procognitive activity. Procognitive effects of the compound could find application for the treatment of memory deficits characteristic of Alzheimer's disease, Parkinson disease, age-associated memory impairment, mild cognitive impairment, brain trauma, schizophrenia and other conditions in which such deficits are observed.
Inhibitors of GSK3β may also find application in the treatment of parenchymal renal diseases (Nelson P J, Kidney International Advance online publication 19 dec 2007) and in the prevention or treatment of muscle atrophy (J. Biol. Chem. 283 2008, 358-366).